Re: [Qemu-devel] [PATCH 11/18] vhost-user: add shutdown support

[Yuanhan Liu]

On Sun, May 01, 2016 at 02:37:19PM +0300, Michael S. Tsirkin wrote:
> On Fri, Apr 29, 2016 at 10:48:35AM -0700, Yuanhan Liu wrote:
> > > But, I
> > > would worry first about a backend that crashes that it may corrupt the
> > > VM memory too...
> > 
> > Not quite sure I follow this. But, backend just touches the virtio
> > related memory, so it will do no harm to the VM?
> 
> It crashed so apparently it's not behaving as designed -
> how can we be sure it does not harm the VM?

Hi Michael,

What I know so far, a crash might could corrupt the virtio memory in two
ways:

- vring_used_elem might be in stale state when we are in the middle of
  updating used vring. Say, we might have updated the "id" field, but
  leaving "len" untouched.

- vring desc buff might also be in stale state, when we are copying
  data into it.

However, the two issues will not be real issue, as used->idx is not
updated in both case. Thefore, those corrupted memory will not be
processed by guest. So, no harm I'd say. Or, am I missing anything?

BTW, Michael, what's your thoughts on the whole reconnect stuff?

        --yliu